This invention relates to apparatus for measuring bipolar fluid flow and, more particularly, to such apparatus especially well suited for use as a spirometer.
My U.S. Pat. No. 3,735,752, which issued May 29, 1973, discloses a spirometer comprising a breath transmission passage in which a venturi is formed, and a flow measurement passage that communicates at one end with the throat of the venturi and at the other end with the atmosphere. Air flow through the breath transmission passage creates a subatmospheric pressure at the throat of the venturi to aspirate air from the atmosphere through the flow measurement passage. Apparatus such as a thermistor bead or a hot wire electrically connected into one arm of a bridge circuit measures the flow rate through the flow measurement passage, which is dependent upon the flow rate through the breath transmission passage. Bias gas, i.e., dry air is preferably supplied to the breath transmission passage to ensure that the moisture does not reach the flow rate measuring apparatus in the flow measurement passage. Such moisture would have a deleterious affect on the accuracy of the measurement and tend to corrode a hot wire.
In the described spirometer, the flow of breath through the breath transmission passage draws air from the atmosphere through the flow measurement passage irrespective of the direction of flow through the breath transmission passage to cool the hot wire or thermistor. As a result, the electrical output from the bridge circuit does not distinguish between inhalation and exhalation.
In the case of children, the total breathing flow rate is so small that the bias gas may appreciably affect the composition of the patient gas supplied by a respirator, particularly the percentage of water in such patient gas. Further, the patient gas inhaled by the patient may contain too much of the patient's breath from his preceding exhalation.